Method of subjecting welded structure to heat treatment

Abstract

A method of subjecting a welded structure to heat treatment in which a tensile yield is produced on an inner surface of the welded structure and a compressive yield is produced on an outer surface of the welded structure by the steps of placing a cooling medium in the interior of the welded structure and heating the outer surface of the welded structure to cause a difference in temperature to exist between the inner surface and other surface of the welded structure. The method further includes the step of locally heating a weld by means of a heating member to improve residual stress in a localized area of the weld and repeating the improvement of the residual stress regarding another localized area of the weld.

What is claimed is: 
     
       1. A method of subjecting a welded structure to heat treatment in which a tensile yield is produced on an inner surface of the welded structure and a compressive yield is produced on an outer surface of the welded structure by the steps of placing a cooling medium in the interior of the welded structure and heating the outer surface of the welded structure to cause a difference in temperature to exist betweem the inner surface and the outer surface of the welded structure, the method comprising the steps of locally heating a weld by a plurality of heating members arranged along the weld to improve residual stress in a localized area of the weld, and repeating the improvement of residual stress in another localized area of the weld so that residual stress in the whole of the weld is improved. 
     
     
       2. A method of subjecting a welded structure to heat treatment as claimed in claim 1, wherein the interval y spacing the plurality of heating members apart from each other if 0<y<Rt (R, the mean radius of the welded structure; and t, the thickness of the wall of the welded structure). 
     
     
       3. A method of subjecting a welded structure to heat treatment in which a tensile yield is produced on an inner surface of the welded structure and a compressive yield if produced on an outer surface of the welded structure by the steps of placing a cooling medium in the interior of the welded structure and heating the outer surface of the welded structure to cause a difference in temperature to exist between the inner surface and outer surface of the welded structure, comprising the steps of locally heating a weld by means of a heating member to relieve stress in a localized area of the weld, and successively moving the heating member to successively heat localized areas of the weld, to thereby provide a continuation of the area where stress has been improved. 
     
     
       4. A method of subjecting a welded structure to heat treatment as claimed in claim 3, wherein the distance x covered by the movement of the heating member is 1/4·L<x<L+Rt (L, the length of the heating member; R, the mean radius of the welded structure; and t, the thickness of the wall of the welded structure). 
     
     
       5. A method of subjecting a welded structure to heat treatment as claimed in claim 4 or 2, wherein the heat generation at end portions of the heating member is reduced below the heat generation at a central portion of the heating member. 
     
     
       6. A method of subjecting a welded structure to heat treatment as claimed in claim 5, wherein the heat generation at the end portions 1/8·L-1/3·L (L, the length of the heating member) of the heating member is reduced to 50-85% of the heat generation at the central portion of the heating member.